1. Field of the Invention
The present invention concerns a method for checking that lenses to be fitted to an eyeglass frame match the contour of the rims or surrounds of the frame.
2. Description of the Prior Art
Eyeglass lenses are manufactured is the form of circular, elliptical or other shape blanks of different sizes which must be trimmed by grinding them to suit the contour of the rims or surrounds of the eyeglass frame to which they are to be fitted.
They are usually trimmed on a grinding machine comprising at least one grinding wheel rotating on a frame, a carriage pivoting on the frame about an axis parallel to the rotation axis of the grinding wheel, a support shaft rotating on the carriage about an axis parallel to the rotation axis of the grinding wheel and which is adapted to receive axially the lens to be ground and a template, and a feeler in vertical alignment with the template mounted on the frame to move transversely to the pivot axis of the carriage.
The template may have a contour identical to that of the rims or surrounds of the eyeglass frame to be fitted with the lens, for example, and when it comes into contact with the associated feeler, which in this case serves as an abutment, it interrupts the movement of the lens into engagement with the grinding wheel due entirely to the weight of the carriage or to the combined effect of its weight and spring, counterbalance or like means. In this way the trimming of the lens at the corresponding location is limited to the required contour.
As an alternative to this the template is round and in this case the associated feeler is acted on by control means which, responsive to the contour of the rims or surrounds of the eyeglass frame to be fitted with the lens, command a displacement causing pivoting of the carriage in one direction or the other so as to adjust the trimming of the lens appropriately at the required locations.
In all cases the practitioner must first choose a lens of adequate size, in other words a lens whose contour is sure to encompass all that of the rim or surround to be fitted with the lens, taking into account the centering which is required when the lens is mounted in the rim or surround.
Various ancillary control devices for this specific purposes have already been proposed.
They are usually simple centering devices which before the lens is attached to the holding block by means of which it is then fitted to the supporting shaft of the grinding machine enable superimposition according to the required centering of an image of the contour of the lens to be fitted and an image of the contour of the rim or surround of the eyeglass frame to which it is to be fitted, so providing a visual check that the lens is suitable for this rim or surround.
As an alternative to this, European patent No 0 379 427 discloses two sensors which are moved over the lens, one on each side, along the contour of the rim or surround of the eyeglass frame to which the lens is to be fitted. A warning signal is sent if the thickness sensed by the sensors along this contour become zero or less than a predetermined limit thickness, so indicating that the selected lens is too small or off-centre.
These devices are in addition to the instrumentation strictly required for trimming the lenses, with attendant cost penalties.
In the absence of such devices the suitability of the lens can be verified by calculation.
However, the calculation is somewhat complicated except for circular contour lenses.
An object of the present invention is a method which provides an economical way of using the grinding machine alone to check the appropriateness of the lens without using any ancillary devices and without requiring any calculations.